\n Deborah Ferguson (UT Austin), Bhavesh Khamesra (Georgia Tech), and Karan Jani (Vanderbilt University)\/LIGO<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n Space-time is being driven apart. Every second that passes, the universe expands faster and faster. What is propelling this dramatic acceleration is an enigma, though \u2013 one scientists have known about, and searched for, for decades. Still, we are no closer to understanding it. We call it dark energy, but we know next to nothing about what it is or where it comes from. Nevertheless, it makes up about 68 per cent of the universe.<\/p>\n It\u00a0would be\u00a0reasonable,\u00a0however,\u00a0to assume\u00a0this mystery\u00a0has nothing to do with\u00a0black holes:\u00a0behemoths\u00a0so gravitationally powerful that once something is\u00a0drawn\u00a0in past\u00a0a\u00a0certain point, it can never escape.\u00a0They\u00a0pull\u00a0matter\u00a0towards\u00a0them, so how could they be driving the\u00a0universe\u2019s\u00a0expansion?\u00a0Yet\u00a0that\u2019s\u00a0exactly what a small group of astrophysicists is\u00a0suggesting.<\/p>\n The story goes like this:\u00a0all matter that falls into black holes goes through a process that turns it into a kind of radiation. This, in turn, exerts a force on the space around it. Such an effect would be too small to notice in the immediate\u00a0surroundings, but\u00a0add together all the black holes in the universe and it starts to mount up\u00a0to something that could be\u00a0pushing everything inexorably away from everything else.<\/p>\n This wild idea began on the fringes, and has appeared in many iterations over the decades. But more and more cosmologists have been paying attention to it over the past few years \u2013 as it turns out to offer a potential explanation for not one, not two, but three mysteries of the universe. \u201cIt\u2019s not fringe any more,\u201d says Kevin Croker, a cosmologist at Arizona State University. \u201cIt\u2019s highly controversial, but it\u2019s not fringe.\u201d<\/p>\n <\/p>\n Black holes offer themselves up as a potential\u00a0source of\u00a0dark energy\u00a0precisely because they are so\u00a0perplexing.\u00a0\u201cMost of the structures in the universe, like galaxies and clusters, have\u00a0very little\u00a0effect on dark energy. But there has always been one\u00a0possible exception,\u201d says\u00a0Niayesh\u00a0Afshordi, a cosmologist at the University of Waterloo in Canada. \u201cBlack holes [after all] are much more mysterious than everything else.\u201d<\/p>\n It all comes down to\u00a0the point at the\u00a0centre\u00a0of a black hole where gravity is so strong that matter is compressed to infinite density.\u00a0Known as an astrophysical\u00a0singularity,\u00a0this\u00a0has always been seen as\u00a0something of a placeholder\u00a0for physics we\u00a0don\u2019t\u00a0yet understand. \u201cNobody believes in a singularity,\u201d says\u00a0Gregory Tarl\u00e9, a cosmologist and astrophysicist at the University of Michigan who is a prominent figure in the study of these cosmologically coupled black holes, so called because they would be coupled with the large-scale behaviour of the cosmos. In reality, he says, something prevents a singularity from forming. \u201cWhat\u2019s going to stop it is if the matter that\u2019s causing this collapse somehow turns into dark energy.\u201d<\/p>\n Nobody knows exactly how it would happen. But Tarl\u00e9 compares it to the very early moments of the universe, when everything was a hot soup of radiation. In the moments after the big bang, the cosmos cooled and much of that radiation coalesced into matter. Inside cosmologically coupled black holes, that process would happen in reverse. This wouldn\u2019t affect their gravitational pull, though, which is based on energy density, not specifically matter.<\/p>\n \u201cIf you try to understand how a single particle of dust can turn into radiation, that\u2019s not known,\u201d says\u00a0Massimiliano Rinaldi, a physicist and cosmologist at the University of Trento in Italy. \u201cBut we assume that it can happen \u2013 this conversion is not as crazy as it sounds.\u201d<\/p>\n For a long time, the consensus has been that black holes can only really affect their immediate surroundings. \u201cThe idea was sort of \u2018what happens in Vegas, stays in Vegas\u2019, but that\u2019s not true,\u201d says Croker, one of the pioneers of the cosmologically coupled black hole concept. \u201cPeople like to throw a causality argument: why could this stuff here affect things that are so far away? But it\u2019s not just one of them, it\u2019s tons of them, and they\u2019re all over the place. It\u2019s this aggregate effect.\u201d<\/p>\n If you threw a bunch of matter into a single cosmologically coupled black hole, it might not affect the cosmos writ large, he says. On the other hand, if you had a fleet of cosmic dump trucks pouring matter into these black holes all over the universe, you could speed up its expansion. It\u00a0is a bit like a balloon filled with many smaller balloons:\u00a0inflate the smaller ones\u00a0and the big one will be forced to expand as well. If these black holes are real, then,\u00a0as a population,\u00a0they must be inextricably tied to the overall structure of the cosmos.<\/p>\n And\u00a0it\u2019s\u00a0not all theoretical, either.\u00a0The first piece of evidence\u00a0that black holes may be cosmologically coupled\u00a0came in 2023 with the revelation\u00a0from Croker,\u00a0Tarl\u00e9\u00a0and their colleagues\u00a0that the small balloons do, in fact, seem to be expanding: black holes across the universe\u00a0appear to be growing at unexpectedly high speeds.\u00a0Even what Croker calls \u201cmaximally boring\u201d supermassive black holes, which\u00a0should\u00a0barely\u00a0be growing at all, are keeping pace with the\u00a0universe\u2019s\u00a0expansion. \u201cIt was the first time we saw something significant that said that once black holes are formed, they create this dark energy, and then the [dark] energy grows as the universe expands,\u201d says\u00a0Tarl\u00e9.<\/p>\n Perhaps the biggest objection to this hypothesis is that we have no idea what cosmologically coupled black holes would look like or how exactly they would behave. \u201cThe problem is that we don\u2019t have a mathematically precise solution that describes these objects \u2013 we have an average,\u201d says Rinaldi. Without that solution, it is impossible to tell, for example, if the behaviour of cosmologically coupled black holes as they merge would match observations we have of that process. \u201cThe task is very, very difficult because the equations are horrible, but there might be a breakthrough at some point \u2013 it just needs time,\u201d he says.<\/p>\n In the few years since the idea was first developed, time and intensive research have shifted it from being something rejected by many serious cosmologists to become something that is at least seen as plausible. One reason for this is that it appears to match up with some puzzling recent results\u00a0from the Dark Energy Spectroscopic Instrument (DESI) in Arizona.<\/p>\n DESI\u00a0is\u00a0measuring\u00a0the locations of millions of galaxies across the universe, building a precise map of how the distances between\u00a0them\u00a0have changed over the course of cosmic history. Those distances allow\u00a0us\u00a0to calculate how fast the universe\u00a0expanded across various epochs.\u00a0And\u00a0over the course of the\u00a0past two years, the first results have been released. They suggest\u00a0that\u00a0dark energy may be weakening over time, which was a bombshell: the standard model of cosmology requires dark energy to be constant. \u201cSeeing the data for the first time, our mouths kind of dropped open,\u201d says Tarl\u00e9. \u201cIt was very clear that dark energy was changing in time.\u201d<\/p>\n But if the effects of dark energy come from cosmological coupling with black holes, the DESI results\u00a0make\u00a0sense. The formation of black holes follows the same trend as star formation, which peaked around 10 billion years ago and has been steadily slowing since then. Not only would this\u00a0explain the lessening\u00a0amount of dark energy\u00a0hinted\u00a0at by DESI,\u00a0it\u00a0would also help account for another major cosmic mystery.<\/p>\n \n Together with dark energy, the pattern of dark matter in the universe (shown above) shapes the structure of the universe<\/p>\n VOLKER SPRINGEL\/MAX PLANCK INSTITUTE FOR ASTROPHYSICS\/SCIENCE PHOTO LIBRARY<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n The Hubble tension relates to a discrepancy between the two main ways of calculating the universe\u2019s expansion, one based on measurements of relatively nearby objects, and another based on using the standard model of cosmology to extrapolate forwards from measurements of light remaining from the big bang. Adding cosmologically coupled black holes to our model of cosmology may not entirely solve this problem, but it significantly eases the tension by providing an explanation for why the two methods deliver conflicting results: the times they probe in cosmic history would have had different rates of expansion.<\/p>\n There are several other proposed explanations for the Hubble tension and the apparent weakening in dark energy, but they tend to rely on exotic hypothetical phenomena beyond our standard understanding of physics. \u201c[The idea of cosmologically coupled black holes] relies upon general relativity and nothing else \u2013 and that\u2019s a plus,\u201d says Rinaldi. Perhaps surprisingly, that makes it a relatively conservative proposition in the context of these two problems.<\/p>\n Now,\u00a0Tarl\u00e9,\u00a0Croker\u00a0and\u00a0a group of colleagues\u00a0have added\u00a0another piece of evidence\u00a0to what they call a \u201cthree-legged stool\u201d of observations that line up with\u00a0their\u00a0predictions.\u00a0This final leg is a bit different from the other two, in that it is a mystery in particle physics. The\u00a0behaviour\u00a0of the universe allows cosmologists to create a budget for how much mass\u00a0it\u00a0contains, which can then be used to calculate the mass of each type of particle.<\/p>\n That\u2019s all well and good, except when it comes to neutrinos, tiny \u2013 but, crucially, not massless \u2013 particles that interact so rarely with other matter that they are sometimes referred to as \u201cghost particles\u201d. Taking into account the new DESI data, neutrinos would need to have a negative mass for the budget calculations to work. As it shouldn\u2019t be allowed to be negative, it must be zero.<\/p>\n But if matter is turning into dark energy inside black holes, that affects the balance of the cosmos. Cosmologically coupled black holes would make room in the mass budget by converting regular matter into dark energy. It turns out they would create just enough leeway for neutrinos to not only have a positive mass, but one that lines up with experimental measurements.<\/p>\n Are these three pieces of evidence enough to fully bring the hypothesis of cosmologically coupled black holes in from the cold? \u201cRight now, the stool of evidence that we\u2019ve offered has the three legs. We think we can sit on it,\u201d says Croker. \u201cOther people in the community may think it\u2019s dangerously janky, but my hope is that, at some point, some other people will jump on this as well.\u201d<\/p>\n That has already started happening.\u00a0The earlier research on cosmologically\u00a0coupled black holes was done by small research groups, each with only a handful of collaborators, but\u00a0the latest paper, on the neutrino masses, has 50 co-authors.<\/p>\n As is always the case with this sort of controversial proposal, what researchers really need are better models \u2013 in this case, solutions to the \u201chorrible\u201d equations \u2013 and more data. The latter, at least, is forthcoming. DESI is still gathering more observations of galaxies, and several other large surveys of the universe are under way. \u201cIt\u2019s a detective story: there is an obvious suspect that is acting very suspiciously and there is an obvious crime,\u201d says Afshordi. With three clues that black holes may be behind the universe\u2019s accelerating expansion, more and more detectives are on the case. \u201cBut, of course, the hard part is making that connection.\u201d<\/p>\n Topics:<\/p>\n<\/section><\/div>\nBlack hole singularity<\/h2>\n
This article is part of our special issue on the crisis in cosmology
Explore the full package here<\/h6>\nEvidence for cosmologically coupled black holes<\/h2>\n
The DESI results<\/h2>\n
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